There are many, many uses for these words. Here are some examples, though: I'm at home; at the store; at the dentist. ("At" shows general location) I'm in the kitchen; in the 2nd aisle of the store; in the waiting room. ("In" often means you can be inside the location and is more specific.) I'm on the couch; on the patio. I'm from the USA; I'm coming home from school. ("From" often shows the place you are leaving or where you originated.)